Downward Causation

all processes at the lower level of a hierarchy are restrained by and act in conformity to the laws of the higher level

This is how Donald T. Campbell (1974) originally formulated the principle of downward causation.
Let us try to clarify what this means. Reductionism can be defined as the belief that the behavior of a whole or system is completely determined by the behavior of the parts, elements or subsystems. In other words, if you know the laws governing the behavior of the parts, you should be able to deduce the laws governing the behavior of the whole.

Systems theory has always taken an anti-reductionist stance, noting that the whole is more than the sum of the parts. In other words, the whole has "emergent properties" which cannot be reduced to properties of the parts. Since emergence is a rather slippery concept, which has been defined in many different ways, most of which are highly ambiguous or fuzzy, I prefer to express this idea with the more precise concept of downward causation.

Downward causation can be defined as a converse of the reductionist principle above: the behavior of the parts (down) is determined by the behavior of the whole (up), so determination moves downward instead of upward. The difference is that determination is not complete. This makes it possible to formulate a clear systemic stance, without lapsing into either the extremes of reductionism or of holism:

the whole is to some degree constrained by the parts (upward causation), but at the same time the parts are to some degree constrained by the whole (downward causation).

Let me illustrate this with an example. It is well-known that snow crystals have a strict 6-fold symmetry, but at the same time that each crystal has a unique symmetric shape. The symmetry of the crystal (whole) is clearly determined by the physico-chemical properties of the water molecules which constitute it. But on the other hand, the shape of the complete crystal is not determined by the molecules. Once a shape has been formed, though, the molecules in the crystal are constrained: they can only be present at particular places allowed in the symmetric crystalline shape. The whole (crystal) constrains or "causes" the positions of the parts (molecules).

The appearance of this "two way causation" can be explained in the following way. Imagine a complex dynamic system. The trajectories of the system through its state space are constrained by the "laws" of the dynamics. These dynamics in general determine a set of "attractors": regions in the state space the system can enter but not leave. However, the initial state of the system, and thus the attractor the system will eventually reach is not determined. The smallest fluctuations can push the system either in the one attractor regime or the other. However, once an attractor is reached, the system loses its freedom to go outside the attractor, and its state is strongly constrained.

Now equate the dynamics with the rules governing the molecules, and the attractor with the eventual crystal shape. The dynamics to some degree determines the possible attractors (e.g. you cannot have a crystal with a 7-fold symmetry), but which attractor will be eventually reached is totally unpredictable from the point of view of the molecules. It rather depends on uncontrollable outside influences. But once the attractor is reached, it strictly governs the further movement of the molecules.

The same principle applies to less rigid, mechanistic systems such as living organisms. You cannot have organisms whose internal functioning flouts the rules of physics and chemistry. However, the laws of physics are completely insufficient to determine which shapes or organizations will evolve in the living world. Once a particular biological organization has emerged, it will strongly constrain the behavior of its components.

For example, the coding of amino acids by specific triplets of bases in the DNA is not determined by any physical law. A given triplet might as well be translated into a multitude of other amino acids than the one chosen in the organisms we know. But evolution happens to have selected one specific "attractor" regime where the coding relation is unambiguously fixed, and transgressions of that coding will be treated as translation errors and therefore eliminated by the cell's repair mechanisms.

A final example from the cultural sphere. Although our basic measuring units (e.g. second or meter) are defined by physical means (e.g. through the invariant length of a particular wave length of a particular type of electromagnetic radiation), the specific choice of unit is wholly arbitrary. The laws of physics impose the constraint that the wave lenght of light emitted by a particular quantum transition as measured in the units we choose must always be the same. However, the choice of a particular unit is not determined by those laws of physics. It is the result of a complex socio-cultural evolution in which different units are proposed for the most diverse reasons, after which one unit is eventually selected, perhaps because it has been used a little bit more frequently by slightly more authoritative sources than the other ones. Once the standard gets established, it becomes a constraint which everybody is supposed to follow. The whole (the socio-cultural system with its standards) determines the behavior of the parts (the measurements made by individuals).